1. Field of the Invention
This invention relates to a spark plug.
2. Description of the Related Art
A spark plug used for ignition of an internal engine of such as automobiles generally comprises a metal shell to which a ground electrode is fixed, an insulator made of alumina ceramics, and a center electrode which is disposed inside the insulator. The insulator projects from the rear opening of the metal shell in the axial direction. A terminal metal fixture is inserted into the projecting part of the insulator and is connected to the center electrode via a conductive glass seal layer which is formed by a glass sealing procedure or a resistor. A high voltage is applied to the terminal metal fixture to cause a spark over the gap between the ground electrode and the center electrode.
Under some combined conditions, for example, at an increased spark plug temperature and an increased environmental humidity, it may happen that high voltage application fails to cause a spark over the gap but, instead, a discharge called as a flashover occurs between the terminal metal fixture and the metal shell, going around the projecting insulator. Primarily for the purpose of avoiding flashover, most of commonly used spark plugs have a glaze layer on the surface of the insulator. The glaze layer also serves to smoothen the insulator surface thereby preventing contamination and to enhance the chemical or mechanical strength of the insulator.
In the case of the alumina insulator for the spark plug, such a glaze of lead silicate glass has conventionally been used where silicate glass is mixed with a relatively large amount of PbO to lower a softening point. In recent years, however, with a globally increasing concern about environmental conservation, glazes containing Pb have been losing acceptance. In the automobile industry, for instance, where spark plugs find a huge demand, it has been a subject of study to phase out Pb glazes in a future, taking into consideration the adverse influences of waste spark plugs on the environment. As a substitute for the conventional Pb glazes, for example, boro-silicate glass or alkali borosilicate glass based glazes have been proposed in JP-A-11-43351 or JP-A-11-106234.
On the surface of the insulator of the spark plug, there are formed markings composed of letters, signs or other images such as manufacturer names, log marks or products numbers. The markings are printed with an ink mixed with a pigment on the insulator surface on which a glaze layer is not formed, followed by coating a glaze on the markings and baking it. In this case, the marking layer is seen through the glaze layer that is made transparent. For distinguishing kinds or forms of the spark plugs easily, the marking layer is often formed by changing a color per each of the kinds or forms.
When the marking layer is formed under the glaze layer, a coloring metallic oxide composing a pigment in the marking layer inevitably causes reaction to a certain degree with components in the glaze layer while baking the glaze. If an existing Pb containing type is employed as the glaze layer, a tint inherent in the coloring metallic oxide is easily maintained, and desired colors can be constantly obtained. However, according to an inventor""s studies, it has been found that if using a glaze where the amount of Pb component in the glaze was controlled to be small like a leadless glaze, it was difficult to adjust the color in the marking layer to be desired tints.
Accordingly it is an object of the invention to offer a spark plug enabling to stably color the marking layer, which is formed under the glaze layer, even if the Pb amount is decreased in the glaze layer.
For accomplishing the above object, a first structure of the spark plug of the invention has marking layer formed on a surface of an insulator and a glaze layer covering the marking layer so that the marking layer can be seen through the glaze layer, and is characterized in that the glaze layer contains 5 mol % or less Pb component in terms of PbO, and kinds and amounts of metallic element components contained in the marking layer are adjusted in such manners that tint of the marking layer seen through the glaze layer is 3 or less in the brightness specified by JIS:Z872 , and 3 or less in the chroma specified by JIS:Z872, otherwise 4 or less in the brightness as well as 2 or less in the chroma.
Desiring the marking layer in particular to color in black, it is important that when observing the marking layer through a reflected white light, the marking layer evenly absorbs the light in each wavelength range of visible spectra such that no outstanding reflection arises by lights of specified wavelength areas, and the level of the whole reflected light is lowered. However, if the Pb amount in the black glaze layer is 5 mol % or lower in terms of PbO, other components than Pb in the glaze layer react with metallic oxides in the marking layer, and the level of light absorption of the specific wavelength by the metallic oxide generating said reaction is changed, otherwise the wavelength of the light absorption is shifted, whereby a balance of the light absorption for coloring the black is lost, and as a result, the marking layer deviates from the black and are easy to color an unwelcome tint. Under such conditions, when distinguishing kinds or types of the sparkplugs by, e.g., colors of markings, the distinguishing will be often difficult. As another realistic problem, there is a case that tint changing in the marking layer is seen to purchasers as xe2x80x9cunreasonable alternation in familiar colors in external appearancexe2x80x9d, so that an inconvenience occurs that products could not always be quickly accepted because of a resistant feeling thereto.
Therefore, in the invention, if kinds and amounts of metallic element components contained in the marking layer are adjusted in such manners that the tint of the marking layer seen through the glaze layer of the Pb amount being 5 mol % or smaller is 3 or less in the brightness specified by JIS:Z8721 (1993) and 3 or less in the chroma, otherwise 4 or less in the brightness as well as 2 or less in the chroma, and even if the glaze layer of the low Pb amount is as mentioned above, externally appearing colors of the marking layer formed under the glaze layer can be constantly recognized as the black.
In the present specification, a measuring method of measuring the brightness and the chroma adopts the method specified in xe2x80x9c4.3 A Measuring Method of Reflected Objectsxe2x80x9d of xe2x80x9c4. Spectral Colorimetryxe2x80x9d in the xe2x80x9cA Measuring Method of Colorsxe2x80x9d of JIS-Z8722 (1994). The brightness and the chroma can be known comparing the result of measuring the brightness and the chroma by the above method with those of standard color chart prepared according to JIS-Z8721.
As a simple method, the brightness and the chroma can be known through visual comparisons with standard color chart prepared according to JIS-Z8721.
The adjustment of the kind or the amount of the metallic element components contained an the marking layer may be performed in accordance with, for example, the following technical concept. What contributes to coloring in the marking layer is mainly several kinds of transition metallic cations (called as xe2x80x9ccoloring metallic componentxe2x80x9d hereafter) ready for causing light absorption by electron transition, and it is assumed that a final coloring of the marking layer is roughly reflected spectra observed as overlapping of light absorption derived from each of metallic cations, in other words, is recognized as mixed condition of colors derived from each of the contained coloring metallic components. It has been found that when the amount of Pb component in the glaze layer goes down, as a result of the inventor""s investigation, specific coloring metallic components such as Cr are easy to make changes in appearing colors (called as xe2x80x9cready discoloring metallic componentxe2x80x9d hereafter) owing to reaction with the glaze layer reducing the Fb amount. In this case, if the ready discoloring metallic compound is too much, only colors of hues after changing by the ready discoloring metallic compounds become intense, and as a whole, this fact results in intensity of deviation of the tint from the black. It is therefore possible to moderate influences in the tint changing by the ready discoloring metallic component if relatively decreasing the amount of the ready discoloring metallic component accompanied with curtailment of the Pb amount in the glaze. On the other hand, if anticipating what is a hue after changing of the ready discoloring metallic component brought about by said curtailment, it is possible to approach to the black the tint as the whole of the marking layer by compounding, as a blackening adjustment component, the coloring metallic component presenting a hue having a large difference from said hue and hue circles (for example, complementary colors).
When adjusting the composition of the marking layer in accordance with the above mentioned technical concept, attention should be paid to the following points. That is, even in a case of the same transition metallic cation, some differences arise in the spectra of light absorption according to the state of its valency electron. For example, the colors of the transition metallic cation may variously change according to the change of its valency electron state causing by the mutual action between the transitional metallic cation and the ions situated surrounding the transition metallic cations, temperatures or atmosphere of baking the glaze. Such phenomena might occur, of course, in the case that other transition metallic cations are situated surrounding the transition metallic cation being the subject of high absorption, and also in the case that the cations of typical metals such as Al or Zn are situated surrounding the transitional metallic cation. In addition, the cation of the typical metal as the latter sometimes performs as a coloring auxiliary component for the coloring adjustment or stabilization.
For distinguishing the tint of the marking layer as the xe2x80x9cBlackxe2x80x9d, a measured value of the chroma of the marking layer should be 3 or lower as an absolute value. Because, if the chroma exceeds 3, it is not easy to erase an impression that the marking layer is apparently colored at seeing an external appearance thereof, irrespective of brightness, and such coloring is heterogeneous from the black. Being over 4, temporarily even if the brightness is very small, the tint is near gray, starting to present an external appearance heterogeneous from the black. As far as being in a range where the brightness is 3 or lower, it may be distinguished as substantially the black until the chroma is around 3, but when the brightness exceeds 3, a coloring is easily sensed to the naked eye by a brightening amount, and therefore with respect to the range where the brightness is 3 to 4, the chroma should be restrained to be 2 or lower.
That the chroma shows not zero but finite values, does not always mean that the xe2x80x9cBlackxe2x80x9d pure in an optical significance is realized. However, if it cannot be confirmed that the chroma presents a tint deviating from the black so far as paying not so much attention, it is difficult to assume that the distinction of kinds by the marking tint is impossible or the tints exceedingly come off from the imaging tints of purchasers. Accordingly, in the object of the invention, tints belonging to ranges of the brightness and the chroma as mentioned above could be regarded as the xe2x80x9cBlackxe2x80x9d.
What the tint of the marking is seen does not depend only on absolute values of the brightness or the chroma, but often depend on a case that an apparent tint is relatively influenced by colors of backgrounds. For example, in case an insulator to be a substrate is formed with a white alumina based ceramic and the glaze is finished nearly to be colorless transparent, the background of the marking will present a white. In such a case, if the tint of the marking layer is mixed with components having colors far off from the black, it is ready for outstanding contrast with the white background. For instance, if the color of the background is white and the like, the brightness and the chroma of the marking layer may be sufficient with said range, but for heightening distinguishability as the xe2x80x9cBlackxe2x80x9d, it is desirable that the brightness as well as the chroma are to be 2 or less. That the color of the background is white and the like denotes in the present description that the chroma is 1 or lower and the brightness is 9 or higher.
The glaze layer of the small Pb amount is sometimes contained with Zn for securing fluidity when baking the glaze. As many of the coloring metallic components in the marking layer are easy to change colors presented by reaction with Zn component, when the glaze layer has Zn component, the effect of the invention is more exhibited. The amount of Zn component in the glaze layer may be selected in the range of 1 to 25 mol % in terms of, e.g., ZnO. Being less than 1 mol %, coefficient of thermal expansion of the glaze layer is too large, and defects as crazing easily occur. Zn component works to lower a softening point of the glaze, and if it runs short, the baking of the glaze is difficult. On the other hand, being more than 25 mol %, the glaze layer is apt to be opaque owing to devitrification. In the latter case, there arise problems that it is difficult to visually recognize the marking layer in the substrate, or the apparent tint of the marking layer becomes gray and easily comes off from the black.
When the marking layer is colored with the black, it is desirable to select one kind or more of Fe, Cr, Co and Mn as the metallic element components to be contained. Among them, Fe and Mn can, even if being singly used, show a tint near the black, and can be effectively used as a base of black group coloring metallic components. Fe and Mn may be used in single or in combination.
If only using one of Fe or Mn, the tint probably falls within a brown group (a red is mixed as a hue), or makes the color irregular depending on the glaze composition, and it will be sometimes difficult to realize an even and stable black tint. Especially in a case of using Mn, easily tinged with a red group, the tint of the whole marking layer is ready for being the brown group. In this case, if compounding one or both of Cr component and Co component as blackening adjustment components, the tint of the marking layer to be obtained is easily adjusted to be black. This effect is particularly large when combining Fe component and Cr component. For example, when the tint of Fe component contains the red group component and the color comes out, since the Cr component trends to present a green group, it may be inferred from the viewpoint of phenomena that the latter serves as the blackening adjustment component and easily realize the black group colors.
Cr component is easy to change the tint when using the glaze composition of a small Pb amount, and in particular when using the glaze containing Zn, Cr component easily shows a tint containing a red of the brown group. Therefore, for suppressing the tint of the red group derived from Cr, it is preferable that the marking layer is composed such that Fe component is 30 to 60 mass % in terms of Fe2O3 and Cr component is 10 to 40 mass % in terms of Cr2O3 for realizing the stable and even black as the tint of the marking layer.
A second structure of the spark plug according to the invention has the marking layer formed on the surface of the insulator and the glaze layer covering the marking layer so that the marking layer can be seen through the glaze layer, and
is characterized in that the glaze layer contains Pb component 5 mol % or less in terms of PbO and Zn 1 to 25 mol % in terms of ZnO, and
the marking layer contains Fe component 30 to 60 mass % in terms of Fe2O3, and Cr component 10 to 40 mass %. in terms of Cr2O3.
If Fe component is less than 30 mass %, it might be difficult to color the marking layer to be a deep black On the other hand, being more than 60 mass %, a margin for containing the blackening adjustment component is made small, and it is difficult to provide the constant and uniform black. In a case of substituting Mn for Fe, a tendency is almost the same, and when using Mn in single or in combination with Fe, a total amount is desirably 30 to 60 mass %. If Cr component is less than 10 mass %, an effect of Cr component as the blackening adjustment is insufficient, and it is difficult to provide the constant and uniform black. Being more than 40 mass %, the tint of the whole marking layer deviates from the black (for example, the marking layer is tinged with the red group color and falls within the brown group), and in turn the brightness and the chrome are easily off from said range. More preferably, the marking layer contains Cr component 10 to 25 mass % in terms of Cr2O3.
The marking layer can contain Co component 10 to 40 mass % in terms of CoO This component trends to color a blue group of the tint far-off from the red, and when using Fe or Mn as the black group coloring metallic component, Co component usefully works as the blackening adjustment component under a condition where the red group color is easy to mix. Being less than 10 mass %, the effect will be insufficient, and the tint of the whole marking layer deviates from the black.
When the red group hue is easily developed owing to a reaction of the Cr component and Zn component, if Co component is added to supplement blue group colors, the tint of the whole marking layer can be brought more nearly to the black. In this case, it is desirable that Cr component and Co component are contained 10 to 40 mass % in total.
The marking layer can further contain Ni component 0.5 to 15 mass % in terms of Ni2O3. Ni component also usefully works as the blackening adjustment component, and for example, when Zn is contained in the glaze layer, Ni component shows a coloring effect of the blue group owing to the reaction with Zn, and a coloring adjustment effect similar to Co can be expected. But being less than 0.5 mass %, the effect will be insufficient, and being more than 15 mass %, the tint of the whole marking layer deviates from the black.
In addition, the marking layer can contain at least one of Al component and Ba component 0.5 to 15 mass % in total in terms of Al2O3 or BaO. These components are effective for accelerating colors of other coloring metallic components contained in the marking layer. But being less than 0.5 mass %, the effect will be insufficient, and being more than 15 mass %, an effect more than that cannot be expected, and the whole amount of the coloring metallic components relatively decrease, so that it will be difficult to color the marking layer in an enough darkness.
Incidentally, aiming at the coloring adjustment, the coloring acceleration, or the homogenization and stabilization of colors other than the above mentioned effects, the marking layer can contain one kind or more of V, Sn, Zn, Ti, Zr, Na, Mg, Si, K, and Ca within the range of 5 wt % in terms of V2O3 SnO2, ZnO, TiO2, ZrO2, Na2O, MgO, SiO2, K2O and CaO, respectively.
Thickness of the marking layer is preferably 1 to 10 xcexcm. Being less than 1 xcexcm, the color of the substrate easily appears, and the tint of the marking layer comes off from the black. On the other hand, being more than 10 xcexcm, irregularities derived from the marking layer are outstanding in the insulator surface to spoil the external appearance.